For anchoring floating installations or subsea equipment to the sea floor, it is known to penetrate the seabed with cylinder shaped skirts. Some of these are known as suction anchors, which are arranged to penetrate the seabed by providing a lower pressure inside the cylinder than on the outside. Others are simply forced into the seabed by gravity. These anchors rely on friction forces between the cylindrical shaped skirt and the seabed.
Patent publication U.S. Pat. No. 6,910,831 describes an anchor which is arranged to penetrate into the seabed by providing a negative pressure inside of it.
In order to provide such a negative pressure the cylindrical shape of the anchor must be closed at the top section. Thus, the main shape of the anchor is the cylindrical side wall and a top plate. Furthermore, it is known to arrange a hatch in the top plate. By opening the hatch, one may penetrate the anchor a distance into the seabed by means of gravity. In addition, when lowering the anchor from the surface, it is advantageous to open the hatch in order to reduce resistance in the water and the added mass that arise when the velocity of the anchor in the water changes. For instance, if the anchor is lowered from a floating installation by means of a lifting crane, the pull in the crane cable may vary when larger waves changes the vertical position or the orientation of the installation. This is particularly crucial when lowering the anchor through the sea surface. In particular, one needs to avoid a slack cable which may disadvantageously affect the crane assembly.
As mentioned, it is known to open a top hatch in order to reduce the added mass. With an open hatch water can flow through the cylindrical shape of the anchor, as well as air when lowering through the sea surface. However, such top hatches of the prior art are disadvantageously small and contributes to reduction of the added mass only to a small extent. Some prior art solutions attempts to solve this by adding more hatches. Thus, some top plates of some anchors have two or three open hatches when lowering the anchor towards the seabed.
By increasing the number of top hatches, the added mass is reduced. However, having a circular top plate with two or three top hatches in it only opens a small part of the total area of the top plate of the anchor. Thus, much added mass remains as a result of the top plate. The present invention seeks to provide a solution to this problem.